1. Field of the Invention
The present invention relates to a test apparatus including an amplitude varying driver circuit which outputs an output signal according to an input signal, and an amplitude varying driver circuit. More particularly, the present invention relates to an amplitude varying driver circuit which outputs an output signal, which is an amplitude-changed input signal.
2. Description of Related Art
Conventionally, a circuit using a differential amplifier as a circuit for changing amplitude of an input signal and outputting the signal is known. Such a circuit controls a current flowing to the differential amplifier, and generates and outputs a potential of the output signal according to the current using a resistor connected to the differential amplifier in series.
In such a circuit, when increasing amplitude varying range, change of the current flowing to the differential amplifier increases, and it is difficult to maintain quality of an output wave between a high amplitude output and a low amplitude output.
Since a transition frequency of a transistor is dependent on a current flowing to the transistor, transition frequencies differ between a high amplitude output and a low amplitude output. Generally, a transistor, which is suitable for the transition frequency optimal for the high amplitude output, is selected. However, in such a case, the transition frequency for the low amplitude output will decrease, and sufficient switching characteristic will not be attained, and a waveform of the high frequency wave will deteriorate.
On the other hand, when the transistor, which is suitable for the transition frequency optimal for the low amplitude output, is selected, the high amplitude output may exceed the limit of the collector current of the transistor.
Therefore, it is an object of the present invention to provide an amplitude varying driver circuit and a test apparatus which can solve the problem of the conventional technology described above. The above and other objects can be achieved by combinations described in the independent claims. The dependent claims define further advantageous and exemplary combinations of the present invention.